In electric power generation, a combined cycle is an assembly of heat engines that work together using the same source of heat, converting the heat's thermal energy into mechanical energy, which, in turn, is used to drive one or more electric generators. In the combined cycle, after a working fluid completes its cycle in a first heat engine, the entropy of the working fluid is still low enough that a second, subsequent heat engine can also extract energy from the waste heat energy of the working fluid of the first heat engine. By combining these multiple streams of work upon mechanical shaft(s) turning electric generator(s), the overall net efficiency of such a system can be increased substantially.
In combined cycle power generating plants, a common combination is a Brayton cycle gas turbine (GT), whose hot exhaust powers a Rankine cycle steam turbine (ST) power plant that drives an electric generator. This is called a Combined Cycle Gas Turbine (CCGT) plant. In a combined cycle power plant, the heat of the gas turbine's exhaust is used to generate steam by passing it through a heat recovery steam generator (HRSG), which is an energy recovery heat exchanger that recovers heat from a hot gas stream. It produces steam that can be used to drive a steam turbine. HRSGs typically include the following components, i.e., an economizer, an evaporator, a reheater and a superheater. Additional steam turbine power can be obtained by increasing the overall steam flow with an additional supplementary fired or “duct” burner in the boiler.
Steam turbines in a multi-shaft combined cycle plant with supplementary firing can become very large due to the additional steam flow from the burner for each of the multiple HRSG units that are present. As such, these steam turbines can require a significantly larger minimum steam flow to successfully start the steam turbine and reach the minimum stable load. In some cases, the steam turbine rotor inertia is simply high due to the design requirements and this too can be difficult to start. A cold (or warm) steam turbine start requires lower temperature steam for stress control. Gas turbine units must therefore be parked at low loads to produce the required steam conditions; however, at low load steam, production is limited and may be insufficient for the steam turbine start up. In accordance with the present invention, supplementary firing of the boiler can be used to add the required steam flow while still maintaining equipment life and low emissions.
A steam turbine sized for (N) multiple, supplementary fired boilers must be able to start with steam from a single (or N−1) gas turbine due to plant operation and maintenance needs. A single gas turbine may not have the exhaust energy needed to produce the required steam flow for a cold steam turbine unit start. Hence, there may be difficulties in starting some steam turbine units in this fashion.
A two gas turbine plant can provide the required steam flow to cold start a steam turbine; however, a two gas turbine plant may not always have two gas turbines available to start a steam turbine, and hence the steam turbine can be prevented from running. Plant owners require the ability to start on a reduced number of gas turbines, i.e., N−1 (or more) gas turbine units, where N is the total number of gas turbine units. Starting capacity can be reduced by unit availability, maintenance needs, grid conditions, emissions limits or simply for an owner's convenience.